I know that this is an old post but I wanted to add my thanks to you Mr. Florance!I have been having battery voltage spike issues for a long time and tried all kinds of things to remedy the situation. Found this post and placed a 47uF cap as you described. PROBLEM SOLVED!! The battV actually looks more noisy now but that's because it's scaled from 12.5 to 14 volts now. It used to autoscale from 7 to 19 ... The car would misfire hard when I had the spikes under boost. Now it's a clean pull straight through

I'm a little bit stumbled. For wiring of most components, like PWM idle valve, relays etc a diode is suggested for protection the mosfets or transistors driving it. However when using mosfets or transistors that are avalanche protected these diode will be less necessary to protect the driver (e.g. irf 540N). But without diodes nasty voltage spike can occur. Why don’t we use diodes in the injector wiring harness?

f3rdy wrote:I'm a little bit stumbled. For wiring of most components, like PWM idle valve, relays etc a diode is suggested for protection the mosfets or transistors driving it. However when using mosfets or transistors that are avalanche protected these diode will be less necessary to protect the driver (e.g. irf 540N). But without diodes nasty voltage spike can occur. Why don’t we use diodes in the injector wiring harness?

With injector drive, you need the clamped spike to close the injector quickly.If you use a rectifier to recirculate the energy at turnoff, the closing time can increase more than 200%.

f3rdy wrote:I'm a little bit stumbled. For wiring of most components, like PWM idle valve, relays etc a diode is suggested for protection the mosfets or transistors driving it. However when using mosfets or transistors that are avalanche protected these diode will be less necessary to protect the driver (e.g. irf 540N). But without diodes nasty voltage spike can occur. Why don’t we use diodes in the injector wiring harness?

To add to what Peter said, the higher the voltage the back EMF is allowed to reach, the more quickly the injectors will close. The active flyback clamp circuit on the v3.0/v3.57 board clamps the voltage to 36V, whereas a 1N400x diode would clamp the voltage to ~14V.

JW3vze4RnR wrote:i am also having this issue. before i opened up my ms again i wanted to be clear on what works and what is speculation..

before i even bother, i am running one 1.3 ohm injector in a tbi do i need to run it with pwm i havent been but my ve table is kind of stretched out from 30 to 212

still not clear on were to attach the "closer to the db37" source for 12v. is that the incoming 12v supply for ms or another line i need to run to a different pin on the db37

im not perfectly clear on the voltages on these capacitors or if it is matters whether its 35v or 50v

here is what i have come up with by reading this and various other threads, i did not add cutting the trace or rewiring the transistor leads as i wasnt going to attempt this yet

photo was borrowed from a mustang forum

Before I did any of that, I would verify that your harness meets the newest MSExtra grounding scheme. Your setup should pass this test:The test is, if you unplug MS box and measure resistance from any sensor's (MAT, CLT, TPS) ground to car/engine ground , you should not have continuity.

If it does pass above, you may find you can run PWM without any other mods.

JW3vze4RnR wrote:peter i have taken the sensor ground test and everything shows no continuity to ground when unplugged from ms..furthermore i can run non pwm injection without noise or voltage spikes.

when switched to pwm injection using all the same hardware i get voltage spikes ranging from 9-17 volts causing my usb adapter and lc1 to shut down.

this is why i was looking to dampen the injection circuit and am looking for clarification that my above posted image is correct at least for the non invasive portions of pwm troubleshooting

Goodyou'll need fairly good sized wire for ground wires from MS to ground. I would use 6 ground wires minimum (most harnesses are coming with #20 gauge wire. I'll run all of them together and crimp them into one yellow ring terminal For the capacitor, I normally will tack a 330uf 25 volt from collector of TIP125 (positive lead of capacitor) to collector of TIP42C. Then bend it towards heatsink and fasten down with hot glue. Should take care of your problem. I run 6 low z injectors. I do have the capacitor added but nothing else.

One other thought; if your PWM is not working correctly (not recirculating current during the PWM time) you can generate a huge amount of noise.

moparmaniac wrote:So I've added a non-polarized .47uF 50V capacitor as you described and am still seeing noise on my tachometer when testing the injector outputs.

I guess I could try a larger capacitor...? Or does the capacitor need to be polarized?

I bought a 47uF (not .47uF) 50V capacitor (polarized this time) and soldered that in as described by Peter.

WORKS AWESOME! That was literally all I did. No lifting traces or anything.

I have absolutely NO noise. My LC-1 is happy (AFR rock solid), all sensor inputs are happy (much more crisp IAT/TPS/etc.), and no more battery voltage sags/spikes. For reference I'm powering 6 low-z injectors on a Mopar 440 v8.